There are no ring species

TRIGGER WARNING: Evolutionary biology.

A while back, when I said in the comments of an evolution post that there were no good “ring species,” a few readers asked me what I meant by that. “What about the salamander Ensatina eschscholtzii? Or seagulls in the genus Larus? Aren’t those good ring species?” My answer was that those had been shown not to be ring species in the classic sense, but there was still one species that might be a candidate: the greenish warbler Phylloscopus trochiloides around the Tibetan Plateau.

But now that one, too, has been struck off the list of ring species, leaving no good cases. Its removal from the class is documented in a new paper by Miguel Alcaide et al. in Nature (reference and link below), in a group headed by Darren Irwin, a professor at the University of British Columbia and including my next-door Chicago colleague Trevor Price.

But first, what is a ring species? Ring species constitute one big and supposedly continuous population in which the attainment of biological speciation (to people like me, that means the evolution of two populations to the point that they cannot produce fertile hybrids were they to live in the same place in nature) does not require full geographic isolation of those populations. Rather, speciation in that continuous population occurs through a gradual spread of the range of the animals, coupled with selection in different places that causes their genetic divergence.

It was long thought by many that for a single species to become two species—to undergo “speciation”—the populations had to be completely separated geographically, so they could evolve along divergent paths without the pollution of genetic interchange that would reduce their divergence. We now know that that isn’t true, and speciation can result even though the populations becoming new species exchange some genes while diverging. Allen Orr and I discussed all this in our 2004 book Speciation, the book that I still consider my proudest accomplishment (see chapters 3 and 4 for the discussion of speciation with gene flow; “ring species” are discussed in chapter 3, pp 102-105).

So a ring species is one case of speciation that is supposed to occur without any geographical isolation.

It works like this: a species expands its range and encounters a roughly round geographic barrier like a valley, the Arctic ice cap, or an uninhabitable plateau. It divides and spreads around the edges of the barrier, so that its range becomes circular as it expands. And as the range begins to form a circle, the populations within it begin to become genetically different as they respond to local selection pressures. But the circle is never interrupted, so while each part of the expanding species becomes genetically different, it still exchanges genes with adjacent populations.

What this causes is a group of populations in which adjacent areas are genetically similar, but become less similar as they become more distant. That’s because the more-distant populations supposedly experience more-different environments, and gene flow between distant populations is attenuated because genes have to flow through all the intervening populations.

At the end, the populations have expanded so far that the ring has “closed”: the species has completely encircled the barrier and the two most genetically diverged populations contact each other. If they are so genetically diverged that they cannot form fertile hybrids, they then appear to be two biological species.

This is a bit of a conundrum because the two “good” species are connected by a chain of populations around the circle, and each population can exchange genes with the adjacent one. This holds all the way around, so, in theory, every population in the ring really belongs to the same species. It’s like breeds of dogs: the Chihuahua can exchange genes with a slightly bigger dog, and that one with a slightly bigger dog, and so on up to the Great Dane. Given this, are Chihuahuas the same species as Great Danes? If you put both species in a kennel together, they couldn’t form hybrids, so you might think “yes, they are different species.” But if you put all breeds of dogs in a kennel, they’d eventually, by mating with dogs of similar size, form a hybrid swarm of mongrels in which Chihuahua and Great Dane genes are found in the “hybrid swarm”.

So even though the populations at the end of the ring behave as two different species when they meet, they’re connected all around the ring by gene flow.

Such species are known as “ring species.” They’re interesting for two reasons. First, they show that you can get the evolution of complete reproductive isolation without geographic isolation. Second it’s a judgment call whether you call them one or two species. If you call them two species, where do you draw the line between the two, given that any adjacent pair of populations around the ring are clearly members of the same species?

There used to be several examples of “ring species” that were staples of evolution textbooks, the most famous being the salamander Ensatina eschscholtzii in California. This species was first worked on by Robert Stebbins but later and most intensively by David Wake of the University of California at Berkeley and his colleagues. It was a classic example supposedly demonstrating all the principles I described above.

An ancestral population of salamanders from what is now Northern California or Oregon was supposed to have spread southward, and then split, with one group expanding its range down the Coast Range, and the other moving east and then expanding southward down along the foothills of the Sierra Nevada. The intervening “center” of the ring was the Central Valley of California, which is grassy, dry, and uninhabitable by these plethodontid salamanders, which need moist habitat. The species thus formed a classic ring, differentiating genetically as both branches moved south. In fact, they became different in color and morphology, and were classified into seven subspecies, as shown in the diagram below.

The ring closed when the ranges encountered each other in southern California, where the subspecies E. eschsholtzii eschscholtzii encountered the long-diverged subspecies E. e. klauberii. These two did not interbreed in nature, and so behaved as different species. Genetic studies demonstrated a long divergence between these, attesting to the “move around the ring” scenario, but also to a lesser divergence between adjacent populations. The ring is shown in the following diagram, along with the distribution of subspecies:

This complex, then, was long regarded as the paradigm of ring species, and was (and is still in places) taught as an example of this form of speciation with gene flow.

Except it’s wrong. That is, it’s not a ring species in the classical sense. Why not? Because genetic studies, done by both Dick Highton at Maryland and then by Wake and his colleagues themselves (references below) also showed that in places around the ring there were sharp genetic breaks, suggesting not a process of continuous gene flow over the 5-10 million years it took to close the ring, but sporadic geographic breaks in the ring, so that the salamanders could differentiate without pesky gene flow from adjacent populations. Some adjacent populations showed very sharp genetic differentiation, implying geographic isolation in the past (Continuous gene flow would not produce such “breaks”.) Finally, geologic work has shown that it is very unlikely that there were two unbroken forest corridors for those millions of years required to produce a ring.

Based on these results, everyone has now concluded that the formation of this “ring” involved sporadic and important episodes of geographic isolation between populations, so it’s not the classic “continuous gene flow” scenario involved in making a ring species. As Wake himself said in his 1997 paper (reference below), “The history of this complex has probably featured substantial [geographic] isolation, differentiation, and multiple recontacts.” (You can read about the Ensatina story in greater detail at “Understanding evolution,” a great site produced by U.C. Berkeley.)

Well, that’s a bummer, but it still shows how geographic isolation by distance can promote reproductive isolation and speciation. Other putative cases of ring species, including gulls in the genus Larus encircling the Arctic, also fell victim to genetic studies, showing that it was very unlikely that they were ever a continuous ring that was geographically uninterrupted.

That left the greenish warbler, which we touted in our book as perhaps the one good case of a true ring species. This species was similar, in that ancestral populations south of the Tibetan plateau were said to have expanded around that uninhabitable plateau (these birds need forests!), meeting north of the plateau in Siberia. Here’s a diagram of the ring with its six present “subspecies” (in different colors), along with the sonograms of each population’s song. The place where the ring was supposed to have joined, and where the populations act as different species, is the crosshatched red/blue area at the top:

Here’s a greenish warbler from Sikkim:

Phylloscopus trochiloides

The original scenario, published by Irwin et al. (2001; reference below) and discussed in our book, suggested that as the species encircled the Tibetan Plateau from the south, the populations diversified in both color and song, with the songs becoming more complex—but in different ways—as the two branches headed north. The evolutionary impetus for song differentiation is not known, but probably involved sexual selection, possibly based on food availability.

By the time the populations met in the north, the male songs were so different that the two populations didn’t recognize each other as members of the same species (i.e., females of one population didn’t respond to the song of males of the other), and so there was no interbreeding, even though there was supposedly interbreeding all along the continuous ring. (The geographic “break” in China is probably due to recent deforestation.)

It was a classic example of ring species, though there was one worry. Mitochondrial DNA (mtDNA) studies in the southwestern part of the ring showed a sharp genetic break between two populations in Kashmir, suggesting that perhaps there was some ancient geographic isolation before the ring began moving. But that differentiation in what is, in effect one gene (the mitochondrial genes are all linked), could have other explanations.

Now, however, genetic analysis of 95 birds and more than 2,000 sites throughout the genome (not just in the mitochondrion), has revealed four genetic clusters around the ring with a sharp intergradation between them (there’s another cluster in the Caucasus, not around the ring). For those of you who know about Bayesian cluster analysis, here’s what the clusters look like around the ring. (Smaller clusters are in the supplementary information.)

Note especially the big break in the southwest, between the yellow and blue populations, and the plot showing the abrupt genetic break between them (lower left), indicating a hybrid zone between populations that were previously isolated geographically. This is in fact in the same place as the worrisome break in mtDNA reported earlier. There are also smaller breaks between Pakistan and Kyrgyzstan, and between Nepal and China (not shown above), but it’s not as clear that those reflect previous geographic isolation rather than simply sampling error or selection with some gene flow.

The authors also found, contrary to previous information, that there are indeed some hybrids (fertile ones, since they backcross) produced where the ring closes at the top, so they aren’t really acting like full biological species.

At any rate, the authors conclude that this case doesn’t correspond to the classical notion of a “ring species,” for that requires no geographic isolation while in this case there almost certainly was some. As the authors note:

Our results indicate that allopatric divergence played a strong role in shaping patterns of genomic divergence during the formation of the greenish warbler ring. Previous findings nonetheless support the idea that geographical isolation has not been the sole driver for the establishment of reproductive isolation; natural and sexual selection also appear to play major roles.

Well, yes, but geographic isolation is never the sole drive of any evolutionary divergence, for something has to make the populations diverge: either selection (natural, sexual, or both) or genetic drift. But it’s good of them to pursue their initial finding more intensively, even if that effaced the cool result of a “ring species” that they reported earlier. But nature is nature, and what happened is what happened. In the end, the authors conclude what I concluded before I saw their first paper: there are no good examples of ring species in nature:

Finally, this study provides meaningful information concerning the interpretation of ring species as prominent illustrations of speciation in the face of gene flow. Earlier analyses of AFLP markers supported the greenish warbler as the last known example in birds of an ideal ring species in which the terminal forms became reproductively isolated despite being connected by gene flow during their whole history of divergence. . It is possible that speciation-by-distance ring species could exist, but their extreme rarity may be explained by the rapidity of Earth’s climatic shifts compared to the time that reproductive isolation takes to occur; over these great spans of time, and as exemplified by the present study, populations are very likely to be temporarily divided.

In other words, perhaps it’s too much to hope for a “true” ring species, for that requires a species’ range to remain unbroken for millions of years—and yet climate and habitat change all the time.

Nevertheless, the results do show a “ring species” of a sort: isolation of two “end” populations of a ring that makes them look like two species, even though all through the ring you don’t see reproductive isolation of adjacent areas. And it shows that speciation can occur despite there having been some gene flow at some times. In nature, populations that form new species must often sometimes exchange genes if they’re not completely isolated by geography (i.e. the finch species that colonized the Galápago), so the dichotomy between “no gene flow” and “pervasive gene flow” may be artificial.

Oh, and there are no good ring species, so don’t go around saying that there are! Mayr concluded the same thing in his great 1963 book Animal Species and Evolution (this book was largely responsible for making me an evolutionary biologist), but he didn’t have genetic data, and he didn’t consider the greenish-warbler case. It’s no great loss, though, that we lack good examples, for ring species didn’t really demonstrate any new evolutionary principles. They showed something we already knew—that reproductive isolation is promoted by anything that reduces gene flow between populations. But they showed it in a cool and novel way.

Another sad story. I used to believe that Monarch and Viceroy were examples of Batesian mimicry, and that Arctic gulls were a ring species. And now, to quote one of your own analogies, they have to join Santa Claus and the tooth fairy.

If we interbred with Neanderthals, the species concept gets a bit blurred round the edges, but so do many others.

One small semantic complaint. You say “geographic isolation is never the sole drive of any evolutionary divergence, for something has to make the populations diverge: either selection (natural, sexual, or both) or genetic drift.” I would prefer to say that isolation is the driver, and drift the mechanism, much as one might say that a difference in kinds of food available can be a driver, and natural selection the mechanism.

I too mourn the loss of Monarch and Viceroy butterflies as an example of Batesian mimicry, although they are an example of Mullerian mimicry. Unfortunately, many textbooks still report the old and wrong view.

I am sure they did. Don’t cats and d*gs have vocal cords? There is some idea that the Neanderthal larynx was higher up, so they did not speak like we did, but they had the same version of the foxP2 gene that we have. This is linked to having language. So they probably had language.

It’s not so much that (though a male Dane biting a Chihuahua in the neck to bring her into heat would probably kill her), but can you imagine any of the Dane genes controlling a trait on a Chihuahua shell… Even a Dane’s ear must mass as much as a Chihuahua leg. Or worse… a Chihuahua digestive system in a Dane?

Biologists can of course use technical terms any way they find convenient, but: what was new to me in this is the fact that the “repeated interbreeding locally” criterion is what is doing a fair bit of work in the argument against ring species here. For example, in the example given with the dogs: one waits several generations (as the chihuahua’s descendents sort of “converge” with those of the great dane’s). To me that suggests at the beginning one sort of had a “web species”, to introduce a term of my own, and then later, by allowing the continual “convergence” returned it to a “single line”. In the classical birds-around-the-arctic putative ring species I remember from somewhere in my distant biology education (20,19 years ago), the ring was taken *at a time*, instantaneously, as it were. Now this is somewhat problematic for other reasons, but it *is* a different situation than I thought what was meant.

It might well be there are no “rings” like that all, but I don’t see why in principle there momentarily couldn’t be. (So in other words, the hypothesis would be to look more!) They may collapse or break into pieces later on, and be unstable given the lack of isolation of many “parts”, but …

Or put it a different way, the dogs *are* a ring species at any moment – just not literally in a ring, and not a ring *of* species, but a species which is in fact “a ring”. I would prefer the term “web species” or the like, though, as the more general term. But that’s an aesthetics of terminology thing.

If a species is an interbreeding population (as per Biological Species Concept and its ilk) it cannot be defined based on an instantaneous snapshot in which no breeding takes place. Is that what you mean by ‘problematic’?

Anyway, the way I prefer to think of these things is that though for practical reasons we have to give names (and assign individuals) to species, they are actually parts of a continuum and not as ‘real’ as species boundaries, places where closely related populations coexist with little or no interbreeding or are separated by a time-transgressive topographic or habitat barrier. There are various kinds of species boundaries…

Although there are no true ring species in the classical sense of speciation with a history of continual interbreeding along the ring population, can we not still say there are ring species by other means? Having historical population breaks such as the case of the salamanders still resulted in populations that act as ‘good’ species in the end, yes? I am hoping to still salvage something from a cool idea, however less elegant.

Hey, how accessible is Speciation to a layman with a general interest in science and evolutionary biology? You’ve mentioned the book many times and I feel I should buy it, as it’s a topic I’d like to delve into a bit more deeply, if it’s anywhere near as readable as Why Evolution is True.

If you’re not trained in evolutionary biology, it would be a hard slog. When I wrote it, several of my friends wanted to buy it and I told them not to, as it was written on a technical level for evolutionary biologists. It’s not a popular book like WEIT. Some of them disregarded my advice and bought the book, and I don’t think any of them ever finished it. So while I’d be delighted if people bought it, you’d probably have to be trained in biology to get the most benefits.

Always a shame to lose a just-so story, but I’d always imagined that simple continuity and gradual divergence over a great distance was a stretch.

I have a pretty good population of Ensatina in my garden — I often find them during the summer drought, sometimes with eggs, a foot or more below the surface in cracks or rodent burrows in my hillside clay soil.

It’s perhaps a bit harsh to describe the ring species idea as a just so story. It was more than that.
It is good though that studies have advanced to give us a better understanding of the evolutionary history of these warblers,gulls and salamanders.

The point of the closed ring is that the endpoint forms are reproductively isolated without being geographically isolated. They act as separate species even though their territories overlap. Without that overlap, you have no evidence of reproductive isolation in the wild.

Are there examples of rings that aren’t closed? Can you sample populations at opposite ends of a range, bring them back to the lab, and see if they breed. I guess some species might be choosy about where they mate?

The problem with that part of the Biological Species Concept is this: what about two populations that could happily interbreed but don’t because they were isolated at the end of the ice age?

In our current warm age they are separate and can only be hybridised by humans artificially carrying one to the other. Are they now two species? But come the next ice age they will be reunited and are one species according to the BSC.

These sorts of dilemmas are exactly why I — granted, an outsider — think that “species” is not the proper terminology for such precision. There’s just too much information that needs to be conveyed than a single binary comparison operator is capable of.

I’m no expert, but the way I’d say it is that while they’re isolated, we simply don’t know whether they’ll ultimately turn out to be one species or two, since that will depend on how long they remain isolated and how much they diverge in the meantime. Whether we can hybridize them in the lab seems irrelevant to that outcome.

Do you have some other species concept in mind that avoids this problem?

No, my point is rather that there is no one size fits all approach. I think that the unattainable ideal is something on the lines of the BSC – two groups of populations that have stopped interbreeding for good – but it comes with a whole number of problems. For example, that we cannot see into the future, that it doesn’t work with fossils, and that by that definition every clonally reproducing individual is its own species.

I deal with all this in my book (chapter 1 and the appendix). Yes, the BSC isn’t perfect, but it’s the concept that I find most useful for the real species question that I (and the founders of the Modern Synthesis) found most interesting: why does nature come in packages that are discrete in a single area, like the various species of birds I see out my window. Why isn’t nature continuous? The answer can only be had by adopting the BSC, for that packaging–the discreteness–is a result of reproductive isolation.

And other species concepts have at least as many problems as the BSC.

There is no “objective” species concept, but at least the BSC enables us to tackle a major biological problem–the one I noted above. Try solving any real biological problem with species concepts like the Phylogenetic Species Concept, whose adherents can’t even agree on.

But I don’t want to argue about this; my views are all in my book, which readers can consult in the library if they wish.

Why isn’t nature continuous? The answer can only be had by adopting the BSC,

I think I see where the disagreement is focussing.

Start with Darwin’s signature tree of life.

If you take a slice through it representing a single moment in time, the discontinuities are obvious and unquestionable — and you, Jerry, have devoted your career to the study of those discontinuities and how they form.

But if you look at it in its entirety over the course of history rather than in a single moment, it most emphatically is continuous. It has to be, or else common ancestry goes out the window and we have branches floating in mid-air.

Darwin’s brilliant isight was to explain both the continuity and discontinuity by inventing his tree of life.

The argument today, I think, is over how narrow or wide a slice of the tree we should be looking at…and that again goes to my suggestion that the term, “species,” isn’t up to the task.

We all know that all the matter in our world that we’re personally familiar with is nothing but up and down quarks and electrons. We also know that that same matter comes in the form of cats and wooden chairs and laptop computers. You can’t meaningfully talk about the one in terms of the other; there is no such thing as a cat electron as opposed to a chair electron, and trying to distinguish between the cat and the chair based on their electrons isn’t going to do you any good either.

I wouldn’t presume to suggest what the particulars of the answer to terminology might be, but I’m quite confident that that’s what’s needed to resolve the controversy and confusion.

The “biological species concept” would be far more palatable if it were more honestly named the “zoological species concept”. It doesn’t apply to biology as a whole — many plants and fungi (and nearly all microbes) don’t work that way at all. The typical zoologist explanation is a handwaving “well, those are just exceptions” — except of course it is the animals which are the exceptions, as they are a tiny fraction of life on earth.

Umm. . . I’m not at all sure you know what you’re talking about. Do you study speciation? If you did, then you’d know that surveys of species distinctness in plants by Loren Rieseberg and others have shown that they’re just as distinct, in terms of the BSC, as are animals. And if you studied biology, you’d know that there are far more species of animals on Earth than plants, even if you include fungi and algae as plants.

If what you mean with the phylogenetic species concept is the idea that species should be monophyletic, that concept is fundamentally incoherent. Sexually reproducing organisms do not have a phylogenetic structure below species level, so claiming that the individuals making up a species are monophyletic is akin to describing a noise level as yellowish.

What many seem to mean is something like “species are indepent genetic lineages as demonstrable by coalescent approaches”, which to me looks like the BSC with a time axis.

If you did, then you’d know that surveys of species distinctness in plants by Loren Rieseberg and others have shown that they’re just as distinct, in terms of the BSC, as are animals.

I am not sure I understand. If the BSC is applied to a group of (sexual) organisms, then the person who applies it will get biological species out of their classification. The question would be whether most botanists would find those species to be useful entities or whether they would consider them to be morphologically and ecologically too heterogeneous, for example.

In many cases I would bet that it boils down to how much occasional gene flow is permitted. As I wrote in another comment, some people believe it should be none, and so they may be rejecting a straw man.

The biological species concept only works with sexual organisms. Many plants and fungi (and nearly all prokaryotes, which are the majority type of life) are not sexual and so the concept utterly fails as a general description of life.

I’m a microbial ecologist; I study the diversity (phylogenetically and functionally) of microorganisms in various environments and conditions. While you can argue that animals have more species than other organisms if you are merely counting “number of monographs published formally declaring a species”, that’s not a very interested definition given that until recently most microbial journals required that organisms be grown in isolated culture before they were formally given a species. That isn’t possible for the vast majority of microbes (the “plate count anomaly” as it is called).

But modern sequencing technology has allowed us to directly see how many species (or species-level operational taxonomic units [OTU]s if you want to be formal about it) are out there. And there’s little question that there are a *lot* more microbial (both eukaryotic and prokaryotic) OTUs than there are macroscopic ones. Yes, there’s arguments that say OTUs are inflated due to sequencing error, but that would presumably inflate the numbers of both microbes and macroscopic OTUs.

You need to read my book because I’ve already discussed this. I’m using YOUR argument that there were more plants than animals. And, pray tell, how do you tell species apart, especially microbial ones, using “modern sequencing technology”. That’s completely arbitrary. I cover a species concept in microbial organisms which, to me, seems far better than the arbitrary difference criterion you use. And, of course, there are far more sexual than asexual species on earth, and even in plants there are mixed mating systems.

You are backpedaling, trying to deny that you said what you said, and I don’t appreciate that. You didn’t mention microbes, you mentioned plants versus animals. I already know that the BSC can’t work in purely asexual microbial species and I have half a chapter on that. So don’t lecture me, please, on what the BSC does or does not cover. I already know that.

I apparently misunderstood the whole concept! I thought the geographic isolation was from the original variety, not from the next-door ones.

I also wonder if the gaps in the “ring” are “missing links” that diligent diggers could find buried if they went looking in the right levels. Ancient history of all kinds is full of gaps. Absence of evidence isn’t evidence of absence!

It would be unlikely to recognize intermediate forms in the fossils, if there are fossils at all. Anatomical differences in the sub-populations are very slight, and in the cases presented here are really about color or songs.
Interesting about your zebra finches. I do not know a lot about birds. But perhaps they are a species with highly variable songs, and /or maybe they mimic things they hear.

It all depends on how the birds recognize conspecifics. Songs are learned (for most species of passerine birds), but some songs are easier to learn than others. And if mate choice is based on some features of song (features we might not find easy to quantify), differences in song might indeed show differences in species. The standard test is to play the recorded songs of one putative species, in the will, to members of the oner putative species. If males don’t charge the playback looking for rivals, or if females aren’t attracted to the playback, we have evidence that they’re separate species.

Or to make that all much shorter: if song is a major isolating mechanism between species, differences in song are ways to recognize speciation.

In the movie Blackfish they talked about dialects in whales. Ever since I saw that I’ve wondered if dialects in birds signal different cultures or family groups. There doesn’t seem to be any evolutionary advantage to having dialects except to identify familial in-groups or clans. Would the female feel more or less drawn to a boy that sounds like her father & brothers? I went into the wrong profession! I’d love to study that.

Any chance that next time we can get a trigger warning for the presence of a trigger warning?

I think the biggest impact of the concept of a ring species has been as a pedagogical tool to help people understand speciation. It might still have some limited utility for that purpose, provided it’s in a context where one can devote equal time to why they don’t actually exist — as you’ve done in this post.

But if the only goal is the original pedagogical one, I think both dogs and ancestral temporal “rings” are much more effective. Richard’s thought experiment of human and chimpanzee girls holding hands with their mothers and the two lines of matrilineal ancestry converging to a common great…great-grandmother a few million years back is an even more powerful one. In that sense, you can consider humans and chimps a “ring,” but a temporal “ring” rather than a geographical one — and such temporal “rings” are exactly what Evolution is all about.

Richard’s thought experiment of human and chimpanzee girls holding hands with their mothers and the two lines of matrilineal ancestry converging to a common great…great-grandmother a few million years back is an even more powerful one.

I agree. And he also (if I remember rightly) talks about how long it would take for generations of humans walking through a door (son followed by father, followed by grandfather, etc.) before you get back to the common ancestor with chimps — and it was an astonishingly short time.

I can give Dawkins’s The Ancestor’s Tale the highest recommendation. Great book; I’m about to re-read it.

Oooh…I should do that math. Picking 5 MYA as the LCA and 10 years for an average generational time (considering the two species as an average and the lateness of the development of delayed sexual maturity for humans), that gives us half a million generations. At one per second, that’s less than six days. A bit long for a commencement ceremony, but not all that long, considering.

At 30 frames per second — one frame for each individual in a movie, we’re looking at under five hours of video for a complete individual-by-individual morph.

Austrofundulus limnaeus occurs from the coastal desert own along the eastern side of Lake Maracaibo down to the Rio Misoa. The end populations look as different from each other as any of the Austrofundulus species. We found intermediate populations intermediate both in morphology and DNA. I think we have a cline there. Unfortunately we did not do hybridization experiments.

Good species of fish hybridize, so you would be very frustrated using your narrow definition of species. There is a Canadian lake with a long history of an intergeneric hybrid swarm of minnows. It is a unique situation, and both species maintain themselves because the hybrids are less fit.

I’m not sure what your definition of “good species” is, but good BIOLOGICAL species can hybridize in nature, so long as the hybrids are very infrequent or sterile. And if they hybridize in the lab (as do lions and tigers in zoos), that doesn’t count.

The situation you describe, where species maintain their integrity in nature, involves hybrid inviability, and that’s a form of reproductive isolation covered by the biological species concept. If two entities maintain their distinctness in nature despite hybridizing, there MUST be some form of reproductive isolation, presumably the inferiority of hybrids, which of course is part of the BSC. We talk about this in our book.

I think all the confusion about species and speciation comes down to an intermingling of subjective and objective criteria.

We have a very similar conundrum with respect to color. Where in the rainbow do you separate red from orange, orange from yellow, yellow from green?

Those terms are immensely useful in all sorts of contexts, but they fail pretty quickly and spectacularly when you try to do anything with precision. And so, color scientists have invented a new and very rich language to use when precision is required. At the extreme, you might indicate the tristimulus value expected of a certain observer in certain ambient conditions when exposed to a stimulus of a certain spectral composition. Most of the time, assumptions of certain standard setups are all you need and you can just use coordinates in the Lab or XYZ color spaces, or even as RGB triplets in a specified (or even assumed sRGB) color space. The latter is how most digital artists think of color.

As an outsider, to me the conflicting definitions of “species” and “speciation” seem akin to arguing over whether a color is reddish-brown or brownish-red. I think some more specific metric would be helpful — these two species share a common ancestor so many generations (years) ago, their genomes have these similarities and differences, their chances of successful hybridization are this-and-such with so-and-so challenges predominating.

I get the impression that that’s the type of language biologists use amongst themselves, and I really appreciate when you do that sort of thing for us, as you did so eloquently in this post.

What I’d like to see is some way to move the discussion beyond arguments of the definition of “species,” with an acknowledgement that the term is indispensable for high-level discussions but falls to something incomprehensible at the level of individuals.

Are you the same species as your parents or children or even siblings? There’s some damned strong reproductive barriers in place in that situation. The absurdity of that question is what I mean by the concept of “species” not being applicable to individuals — but populations, of course, where the term is indispensable, are composed of individuals. How many pebbles make a pile? Is a single water molecule wet? What’s the difference between reddish-brown and brownish-red?

Well, part of the problem is that different people define the BSC in different ways. Yes, you have repeatedly said that some limited gene flow is allowed. But I have run into other colleagues who have a much stricter view.

Conversely, there are many more people who do not find the BSC useful for their groups of interest, and not only in fossils (too dead to mate) or clonally reproducing species.

What, for example, about two plant populations with different ploidy levels? The tetraploids are probably entirely unable to move genes back into the diploids, so in that direction we observe lack of gene flow. But the diploids are still free to undergo separate polyploidisation events, making their offspring members of the tetraploid population and thus leading to occasional but potentially significant one-way gene flow…

I like your hybrid swarm explanation of why Chiwawas are considered to be the same species as Great Danes even though they are incapable of interbreeding. That’s something I’ve always found confusing… But would they still be the same species if there were no other breeds that enabled the cross propagation of genes within the pool?

And all dogs are now regarded as a subspecies of Canis lupus, no? Great Danes breeding with wolves is easy to imagine; chihuahuas, not so much. Especially not ŵ a chihuahua bitch! (This is a problem with some Panthera crosses, iirc.)

In the early sixties I worked in the Dobzhansky lab, along with some other undergraduates, on mating preferences in Drosoph8ila paulistorum populations collected from sites encircling Brazil. I recall that Dobzhansky called the populations, sibling species in statu nascendi since reproductive isolation increased with distance between sites. Would that have merited calling the group a ring species??

I’m leaving this comment just because I know our host sometimes despairs over posts about cats getting many more views than posts about science. Buck up, Dr. Coyne: many of us are here just for this sort of thing!

(…okay, I have no evidence for that. But I’m certainly here for it, and the chances of me being in the norm are of course higher than of me being an outlier, so I’m sure there are more folks like me.)

Hm. Your definition of a ring species is certainly very strict. I was assuming that the major point of the concept was that speciation can be gradual, that you could have a line of individuals of which all neighbors could interbreed but the ones at the two ends couldn’t.

Whether the two ends have actually come around to meet geographically or whether there are absolutely zero breaks along the chain did not seem so important to me as long as things along the chain can interbreed – not least because geographic isolation appears to be a matter of degree. (Perhaps less so in salamanders that don’t move a lot, but clearly in birds or plants whose pollen are carried around a lot.)

I have a slide on Ensatina in my lecture. After reading this post I will be more circumspect when discussing speciation in a few weeks but still the fact remains: there are cases where we must realise that “different-species-ness” sinks into a grey area, such as in the example of the dogs which could be seen as two species if all intermediates were killed off.

That doesn’t mean that there are no species, just like the grey area between a toddler and an adult doesn’t mean that there are no adults. But it seems important to me to get the problem across because nature does not do us the favour of always being about neat little boxes.

A thought experiment. A large island/small continent with no canids is seeded with populations of Great Danes and Chiuauas. The island is revisited 500 years later. What might we find? Of the several possibilities, which do you think most likely?

I think the picture after 500 years will not be much different from the picture after 100 years, and that it will likely include the extinction of various native species of ground-dwelling birds, reptiles, and mammals.

Beyond that, a lot depends on initial conditions. If prey is plentiful, and the dogs are introduced together, as a single mixed group, then I think we can expect some hybridization between the two breeds. (I’m not one of those who think that such hybridization is obviously impossible. As I child I witnessed the successful hybridization of my miniature collie with the Saint Bernard up the street. Where there’s a will, there’s a way.)

On the other hand, if prey is scarce, and the Danes don’t meet the Chihuahuas until after they’ve decimated the local fauna, then things don’t look too good for the Chihuahuas.

Of course, if the small, dogless continent we’re talking about is Antarctica (I don’t know of another one), then I wouldn’t expect any survivors of either breed.

I don’t agree that the history of a ring species is the point at all. Sure, if your professional interest is in the process of speciation, you’ll come from that angle.

But the ring species concept is best employed, in my opinion, to demonstrate the continuous nature of evolution. If you can construct a ring of populations that all interbreed except at one point, that’s a true and proper ring species. How it got that way is quite besides the point.

I totally agree Thanny.
Anyway, what I have always loved about ring species is that the example is a perfect rejoinder to the argument that many id proponents have, that there is no example of speciation in higher species which can be seen “taking place”. Bringing up ring species has always stopped this argument in its tracks.
A sad loss to the ease of winning evolutionary arguments with ID-oriented ignoramuses if examples such are “lost”!

Also disappointed, though not terribly surprised… incidentally, one of my colleagues was using ring species as an example of obvious speciation in a chapter on evolution in a pop-sci book (not yet finished), and now he’s scrambling to find a replacement example. If anyone has a better example of speciation in modern animals that has been observed directly by humans, I’d be most obliged…

I’m somewhat upset here.
I always used the larus gulls (and to a lesser degree the ensinata) as an argument that there is no difference between micro and macro evolution in discussions with the religious and other evolution deniers. “What can be done in space can even more easily done in time”, that line of arguing.

Do these gulls have regular interbreeding at present between these populations that apparently had some other barrier in the past?
If so, the argument stands (regardless of causation) meseems, if not, I’ll have to scrap it. Obviously, I would not want to use a void argument. Do I have to throw it out?

I’d like to add that I’ve been to the Zealand (old, not New) coast. The herring gull and lesser black backed gull are clearly different species there, easily recognizable as such. If any hybridization takes place it is well hidden from the laymans eye.

Thank you for this Jerry. I never understood ring species very well. But I always thought the Ens. salamander was a case of geography isolation. In these scenarios where species split geographically I just can’t understand how a ring species could exist.

Peter Hadfield uses ring species to puncture creationists’ concept of “kinds” here. http://youtu.be/l-ilMYc5xdQ
However I don’t think the revision of there being no “true” ring species seriously undermines his argument.

At first I was worried that I had lost a useful teaching tool that facilitated student discussion and critical thought about species, speciation, gene flow, and hybridisation. But I think Alcaide et al.’s results might actually make the greenish warbler example even more pedagogically helpful by demonstrating the relationship between useful — but greatly simplified — conceptual models, versus empirical data that are necessarily more messy as a result of complex and stochastic evolutionary and ecological processes. As a bonus, this nicely demonstrates the importance of testing even the most attractive of stories.